Modern oil field operations demand a great quantity of information relating to the parameters and conditions encountered downhole. Such information includes characteristics of the earth formations throughout the wellbore, and data relating to the size, direction and configuration of the wellbore. The collection of information relating to conditions downhole is performed by several methods including wireline, measuring-while-drilling (MWD) and logging-while-drilling (LWD).
In wireline, a probe or “sonde” is lowered into the wellbore after some or all of a well has been drilled. The sonde hangs at the end of a long cable or “wireline” that provides mechanical support to the sonde and also provides an electrical connection between the sonde and electrical equipment located at the surface of the well. Various parameters of the borehole and/or the earth's formations are measured and correlated with the position of the sonde in the wellbore as the sonde is pulled uphole.
In MWD and LWD, the drilling assembly includes sensing instruments that measure various parameters as the formation is being drilled. These while-drilling techniques transmit to the surface real-time information relating to the conditions downhole, and as such allow for an immediate response to wellbore conditions. Various telemetry techniques are used to transmit the information to surface, such as mud pulse telemetry.